Molecular Transformation of Crude Oil Contaminated Soil after Bioelectrochemical Degradation Revealed by FT-ICR Mass Spectrometry

Dual-purpose elemental sulfur for capturing and accelerating biodegradation of petroleum hydrocarbons in anaerobic environment

Hydrophobic organic pollutants in aqueous environments are challenging to biodegrade due to limited contact between microorganisms, the pollutants and the electron acceptor, particularly under anaerobic or anoxic conditions. Here, we propose a novel strategy that uses inexpensive, dual-function elemental sulfur (S0) to enhance biodegradation. Using petroleum hydrocarbons as the target pollutants, we demonstrated that hydrophobic and nonpolar S° can concentrate hydrocarbons while simultaneously serving as an electron acceptor to enrich hydrocarbon-degrading bacteria. The permeable reactive barrier filled with S0 effectively removed petroleum hydrocarbons. In addition to rapid adsorption, we discovered, for the first time, that petroleum hydrocarbons underwent efficient biodegradation through the reduction of S0. Specifically, n-alkanes were degraded by 80 % to 90 % and polycyclic aromatic hydrocarbons by 40 % to 95 %. These degradation rates were 17 % to 30 % and 26 % to 43 % higher, respectively, compared to those observed without S0. Consecutive subcultures combined with untargeted metabolomics analysis revealed that bacteria capable of dissimilatory sulfur reduction enhanced the fermentation process. These bacteria provided electrons to the metabolic network, which facilitated the mineralization of petroleum hydrocarbons. Our findings highlight the significant potential of S° for removing hydrophobic organic pollutants in oxygen-free environments, demonstrate the feasibility of integrating adsorption, biodegradation, and electron supply to enhance pollutant removal.

Assessment of the Migration of Polar Compounds from Petroleum-Contaminated Soil Using a Column Leaching Experiment

When petroleum leaks into soil, the polar compounds exhibit strong biological toxicity, causing serious damage to soil animals, plants, and microorganisms and potentially threatening human health. However, the systematic comprehension of the migration of polar compounds in petroleum-contaminated soil remains limited. Herein, we employed elemental analysis, stable carbon isotope analysis, and high-resolution mass spectrometry techniques to study the migration of polar compounds in petroleum-contaminated soil using a column leaching experiment. The results indicate that petroleum migration ability in soil is limited, and the compounds are primarily concentrated in the soil above 40 cm. The C/N, C/H, and δ13C ratios of organic matter in soils are highly affected by petroleum contamination. Meanwhile, the different compound classes show varying migration abilities, with N1 and N1O1 compounds exhibiting stronger adsorption capacity on soil, while oxygen-containing compounds are more likely to migrate with water to deeper soil. Additionally, molecular polarity, unsaturation degree, and size are key factors affecting the migration of polar compounds in petroleum within the soil. This simulation experiment offers valuable insights into comprehending migration of polar compounds in petroleum-contaminated soil and their potential impacts for soil ecological environment.

An overview of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils

The global problem of petroleum contamination in soils seriously threatens environmental safety and human health. Current studies have successfully demonstrated the feasibility of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils due to their easy implementation, environmental benignity, and enhanced removal efficiency compared to bioremediation. This paper reviewed recent progress and development associated with bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils. The working principles, removal efficiencies, affecting factors, and constraints of the two technologies were thoroughly summarized and discussed. The potentials, challenges, and future perspectives were also deliberated to shed light on how to overcome the barriers and realize widespread implementation on large scales of these two technologies.

Effects of thermal desorption on ecotoxicological characteristics of heavy petroleum-contaminated soil

This study comprehensively evaluates the ecotoxicity of high-concentration heavy petroleum (HCHP)-contaminated soil before and after thermal desorption (TD) remediation at different temperatures and times. The results showed that the detoxification of contaminated soil was effectively achieved by extending the remediation duration at 400-600 °C. After treatment at 400 °C for 60 min, the toxicological indicators including bioluminescence EC₅₀ (acute toxicity), seed germination ratio (Gr) and plant biomass of Brassica juncea (subacute toxicity), and diversity of the microbial community (chronic toxicity) reached a maximum. The value of the SOS-Induction Factor (SOSIF), characterizing genotoxicity was below 1.5, indicating that it was non-toxic. Pearson's correlation analysis illustrated that the water-soluble fraction (WSF), ALK1-3 and ARO1-3 of petroleum hydrocarbons were the primary sources of ecotoxicity. Notably, although the total ratio of petroleum removed from the soil reached 87.26 ± 4.38 %-98.69 ± 1.61 % under high-temperature thermal desorption (HTTD, 500-600 °C), the ecotoxicity was not lower than that at 400 °C. The pyrolysis products of petroleum macromolecules and extreme changes in soil properties were the leading causes of soil ecotoxicity following HTTD. The inconsistency between the removal of petroleum pollutants and ecological health risks reveals the significance of soil ecotoxicological assessments for identifying TD remediation endpoints and process optimization.

Using Potential Molecular Transformation To Understand the Molecular Trade-Offs in Soil Dissolved Organic Matter

Understanding the chemical composition and molecular transformation in soil dissolved organic matter (DOM) is important to the global carbon cycle. To address this issue, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was applied to investigate DOM molecules in 36 paddy soils collected from subtropical China. All the detected 7576 unique molecules were divided into seven compound groups, and nine trade-off relationships between different compound groups were revealed based on principal component analysis and Pearson's correlation. An optimized method was developed to evaluate all potential molecular transformations in DOM samples. The concept of thermodynamics was introduced to evaluate the identified molecular transformations and classify them as thermodynamically favorable (TFP) and thermodynamically limited (TLP) processes. Here, we first tried to understand the molecular trade-offs by using the potential molecular transformations. All the nine trade-offs could be explained by molecular transformations. Six trade-offs had bases of biochemical reactions, and the trade-off-related direct transformations could explain the content variations of carbohydrate-like, condensed aromatic-like, tannin-like, and lignin-like compounds in TLP. More reasonable explanations existed in the TLP rather than TFP, which demonstrated the critical role of external energy in the molecular transformation of soil DOM.

Molecular transformation and composition flow of dissolved organic matter in four typical concentrated leachates from the multi-stage membrane system

Concentrated leachate (CL), characterized with high content salts and compositional complexity of dissolved organic matter (DOM), is difficult to degrade. Understanding the CL from molecular insight level is the requirement for further disposal based on their components. Here, typical CL samples were collected from the multi-stage membrane separation process in a large-scale leachate plant, including nanofiltration (NF), primary ultrafiltration (PUF), secondary nanofiltration (SNF), and reverse osmosis (RO). More than 95% of DOM was removed from raw CL, of which about 3/4 flowed into PUFCL and 1/5 flowed into SNFCL. DOM with macro-molecular weight (>500 Da, 30.46%) and highly unsaturated compounds (double-bond equivalents >15) were detected in PUFCL. Nearly half of DOM was CHO-only compounds (42.04%) in SNFCL. PUFCL was abundant in heteroatom species with higher-order oxygen (O ≥ 10), which was coincident with the trend of humic substance distribution (humic substance >1/2). Based on these properties results, advanced oxidation processes, such as ozonation, might be the right process for SNFCL rich in heteroatom species with low-order oxygen (O < 10). Abundant disulfides (S₂O_2-6 classes, 20.19%) and monovalent salts existed in ROCL, which should be removed from the system. These findings might provide basic information for the treatment of CLs from different membranes.

Remediation of soil polluted with petroleum hydrocarbons and its reuse for agriculture: Recent progress, challenges, and perspectives

Petroleum hydrocarbons (PHs) are used as raw materials in many industries and primary energy sources. However, excessive PHs act as soil pollutants, posing serious threats to living organisms. Various ex-situ or in-situ chemical and biological methods are applied to restore polluted soil. However, most of the chemical treatment methods are expensive, environmentally unfriendly, and sometimes inefficient. That attracts scientists and researchers to develop and select new strategists to remediate polluted soil through risk-based analysis and eco-friendly manner. This review discusses the sources of PHs, properties, distribution, transport, and fate in the environment, internal and external factors affecting the soil remediation and restoration process, and its effective re-utilization for agriculture. Bioremediation is an eco-friendly method for degrading PHs, specifically by using microorganisms. Next-generation sequencing (NGS) technologies are being used to monitor contaminated sites. Currently, these new technologies have caused a paradigm shift by giving new insights into the microbially mediated biodegradation processes by targeting rRNA are discussed concisely. The recent development of risk-based management for soil contamination and its challenges and future perspectives are also discussed. Furthermore, nanotechnology seems very promising for effective soil remediation, but its success depends on its cost-effectiveness. This review paper suggests using bio-electrochemical systems that utilize electro-chemically active microorganisms to remediate and restore polluted soil with PHs that would be eco-friendlier and help tailor-made effective and sustainable remediation technologies.

Insights into the molecular transformation in the dissolved organic compounds of agro-waste-hydrochars by microbial-aging using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Hydrochars-based dissolved organic matters (DOM) are easily available to organisms and thus have important influence on the biota once applying hydrochars as environment amendment. Thus, positive modifications on molecular composition of DOM is indispensable before hydrochars application. In this study, the impacts of microbial-aging by anaerobic fermentation on DOM of agro-waste-hydrochars was systematically assessed. Results revealed that microbial-aging caused lower DOM release but higher DOM molecular diversity. Moreover, microbial-aging resulted in the production of more biodegradable compounds, including lipids and proteins, and reduced the aromaticity of DOM. The highly oxygenated molecules (O/C > 0.6) were shifted into lower-order ones in the hydrochars-based DOM, suggesting the transformation of hydrophilic compounds into hydrophobic ones. Additionally, microbial-aging promoted the degradation of phenols by 99.0-98.9%, phenolic acids 37.8-73.5%, and polycyclic aromatic hydrocarbons by 83.4-90.4% in hydrochar-based DOM. Overall, this study demonstrates that microbial-aging changes the molecular characteristics of hydrochars-based DOM in a positive manner.

Moisture retention extended enhanced bioelectrochemical remediation of unsaturated soil

Bioelectrochemical systems (BESs) have demonstrated great promise in augmented biodegradation of petroleum hydrocarbons in water-saturated soils. However, bioremediation of unsaturated soil in vadose zone has been a challenge due to poor mass transfer and low conductivity. This study proposed a moisture retention layer (2 cm thickness) around the BES anodes to enhance soil remediation under unsaturated conditions. The active soil BESs (closed circuit) includes two reactors with anodic moisture-retaining layers of soil-polyacrylamide hydrogel (SHB) and graphite granule-polyacrylamide hydrogel (GHB) mixtures, and another reactor filled with only soil (SB) without moisture-retaining layer. An open circuit SB was served as a control to simulate natural attenuation. This study demonstrated for the first time that moisture retention layers around the BES anodes could significantly extend and enhance hydrocarbon degradation in vadose zone soil. Results showed that SHB reactor could maintain 43-100% longer duration for electricity generation than other reactors. Correspondingly, SHB showed the best removal (average 21-37%) of total petroleum hydrocarbon (TPH) in spatial distribution, which was ~91% and ~164% higher than other BESs and control, respectively. This study demonstrated that by using low-cost and environmentally friendly hydrogel, BESs could become a viable remediation method for vadose zone soil.